High-speed steel containing chromium tungsten molybdenum vanadium and cobalt

ABSTRACT

THIS INVENTION RELATES TO A TOOL STEEL CONSISTING ESSESTIALLY OF, IN WEIGHT PERCENT, CARBON 1 TO 1.4, CHROMIUM 4 TO 6 VANADIUM 1 TO 1.5 TUNGSTEN 7.5 TO 13, MOLYBDENUM 3.5 TO 7, COBALT 9 TO 15, NITROGEN AT LEAST ABOUT .03 AND PREFERABLY .03 TO .08, AND THE BALANCE IRON. THE IVENTION ALSO RELATES TO A TOOL STEEL COMPACT OF THIS STEEL PRODUCED BY A POWDER-METALLURGY TECHNIQUE ALSO IN ACCORDANCE WITH THIS INVENTION. THE TOOL STEEL ARTICLE IS CHARACTERIZED BY A COMBINATION OF GOOD CUTTING PERFORMANCE AND MACHINABILITY.

Dec. 14, 1971 s. STEVEN 3,627,514

HIGH-SPEED STEEL CONTAINING CHROMIUM, TUNGSTEN, MOLYBDENUM, VANADIUM ANDCOBALT Filed May 7, 1969 3 Sheets-Sheet 1 M GNIFICATION IOOO X P1 E: 1B.

MAGNIFICATION IOOO X INVENTOR. GARY STEVEN Attorney Dec. 14, 1971 STEVEN3,627,514

HIGH-SPEED STEEL CONTAINING CHROMIUM, TUNGSTEN, MOLYBDENUM, VANADIUM ANDCOBALT Filed May 7, 1.969 5 Sheets-Sheet 2 -Graff Grain Size 72, SnyderO l NVENTOR.

GARY S TEVEN Attorney Dec. 14, 1971 STEVEN 3,627,514

HIGH-SPEED STEEL CONTAINING CHROMIUM, 'IUNGSTI'IN,

MOLYBDENUM. VANADIUM AND COBALT 3 Sheets-Shoot 3 Filed May 7, i969Snyder-Groff Grain Size INVE'NTOR.

GARY STEVEN At torney United States Patent HIGH-SPEED STEEL CONTAININGCHROMIUM,

TUNGSTEN, MOLYBDENUM, VANADIUM AND COBALT Gary Steven, Mount Lebanon,Pa., assignor to Crucible Inc., Pittsburgh, Pa. Filed May 7, 1969, Ser.No. 822,672 Int. Cl. C22c 39/14 U.S. Cl. 75-126 C 1 Claim ABSTRACT OFTHE DISCLOSURE This invention relates to a tool steel consistingessentially of, in weight percent, carbon 1 to 1.4, chromium 4 to 6,vanadium 1 to 1.5, tungsten 7.5 to 13, molybdenum 3.5 to 7, cobalt 9 to15, nitrogen at least about .03 and preferably .03 to .08, and thebalance iron. The invention also relates to a tool steel compact of thissteel produced by a powder-metallurgy technique also in accordance withthis invention. The tool steel article is characterized by a combinationof good cutting performance and machinability.

For all tool steel articles for cutting applications, it is desired tohave a combination of machinability and good cutting performance. Thisis a somewhat difficult combination to achieve in that for good cuttingperformance the alloy from which the tool steel article is made must becharacterized by high hardness. On the other hand, the harder thematerial, the more difficult it will be to machine. In addition, andmore specifically, this desired combination of properties is affected bythe carbide size and distribution within the steel. A fine, evendispersion of adequate carbides will provide the required hardness andthus tool life. However, to achieve substantial carbide formation it isnecessary to employ high austenitizing temperatures, on the order of2200 F., so that the carbide formers present in the alloy go intosolution and are thus available to precipitate as carbides upontempering. The higher the austenitizing temperature, the greater will bethe amount of carbide formers in solution, and thus the amount ofcarbides formed upon tempering. It is known, however, that the use ofhigh austenitizing temperature results in grain coarsening of the alloyand excessive carbide growth and agglomeration. Grain coarsening andexcessive coarsening of carbides, as is well known, impair cuttingperformance of tool steel articles.

It is accordingly the primary object of this invention to provide a toolsteel that overcomes the above-described disadvantages in that it ischaracterized by a good combination of machinability and cuttingperformance.

A more specific object of the invention is to provide a tool steel thatmay be austenitized at the high temperature required to take the carbideformers present in the material into solution, without causing attendantgrain coarsening.

Another more specific object of the invention is to provide a tool steelalloy wherein a good combination of machinability and cutting perforanceis achieved by a critical combination of a controlled nitrogen contentin combination with specific carbide formers wherein a fine, uniformcarbide distribution is maintained even in the presence of highaustenitizing temperatures.

Another related object of the invention is to provide a tool steelcompact produced in accordance with a powderice metallurgy process thatresults in said article having a desired combination of goodmachinability and cutting performance resulting from the presence of afine, uniform carbide distribution throughout the compact.

These and other objects of the invention, as well as the completeunderstanding thereof, may be obtained from the following descriptionand drawings, in which:

FIGS. 1A and 1B are photomicrographs of a steel in accordance with thepresent invention and a conventional tool steel, respectively, whereinthe effect of the invention is shown in respect to the carbide form,size and distribution; and

FIGS. 2A and 2B are three-dimensional plots of grain size vs.austenitizing temperature and carbon content, and again size vs.austenitizing temperature and carbon plus nitrogen content,respectively.

The tool steel of the invention consists essentially of, in weightpercent, carbon 1 to 1.4, chromium 4 to 6, vanadium 1 to 1.5, tungsten7.5 to 13, molybdenum 3.5 to 7, cobalt 9 to 15, nitrogen at least about.03 and preferably .03 to .08, and the balance iron. In accordance withthe invention, this steel is used in the form of a powder of about --8mesh U.S. Standard. This powder is placed in a. metal container, whichis gas tight. The container is heated to an elevated temperature inexcess of about 2000 F. and its interior is pumped to a low pressurewhereupon the gaseous reaction products and principally those resultingfrom the reaction of carbon and oxygen are removed. Upon removal of thegaseous reaction products and while the container is at low pressure andelevated temperature it is sealed against the atmosphere, andtransferred to a compacting apparatus. Compacting may be by mechanicalapparatus wherein the container is placed in a die and a ram is insertedto compact the container and charge. Alternately, the container may beplaced in a fluid-pressure vessel, commonly termed an autoclave, where afluid pressurizing medium, such as helium gas, may be employed toprovide the desired compacting. In any event, however, compacting iscompleted prior to the charge cooling below a temperature of about 1900F., and during the operation a compacted density greater than about isachieved. After compacting, conventional forming and machiningoperations are performed on the compact, during which a density of isachieved, to produce the desired final tool steel product. To achievethe required nitrogen content in the alloy, in accordance with thepresent invention, such may be either included in the melt or,alternately, nitrogen in gasous form may be introduced to the container,as above described, after outgassing and prior to compacting. In thismanner, the charge of powered metal in the container will be nitrided tothe desired nitrogen level in accordance with the invention.

The carbon content of the alloy, as above disclosed, must be properlybalanced against the carbide-forming elements, such as vanadium,tungsten and molybdenum, to produce the carbide precipitation uponcooling from austenitizing temperature required to prevent softeningduring subsequent annealing. Of the carbide formers, vanadium functionsto produce carbides that have been found to be wear-resistant and thuscontribute greatly to the tool life of articles made from the alloy.However, if too much vanadium is used these wear-resistant carbides makethe steel difficult to machine and grind. Tungsten, on the other hand,provides carbides that retain hardness at high temperature, principallybecause they do not appreciably or substantially grow and agglomerate athigh austenitizing temperatures and, therefore, grain coarsening of thealloy is retarded. Molybdenum acts in the same manner as tungsten withrespect to carbide formation, except that tungsten is critical for thepurposes of pre- 4 duced. In addition to the Rex 71 P/M steels listed inTable I, two additional compacts with similar compositions except forhaving nitrogen contents of .003 and 017% were prepared.

TABLE I.-OOMPOSITION 1 Chemical composition, percent AISI Steel type CCr W Mo V Co N Rex 71 P/M 1. 20/1. 25 4. /4. 50 10. 0/10. 5 5. 00/5.50 1. 15/1. 40 12. 00/12. 50 0. 03/0. 08 Rex 49 M41 1.10 4. 25 6.75 3.75 2.00 5.0 Rex M42 M42 1 10 3. 75 1. 5 9. 5 1. 8.0 Maxicort 2 1 25 4.25 10.5 3.75 3. 25 10. 5

1 All steels contain nominally 0.3% Mn, 0.3% Si, 0.025% S max. and0.025% P max.

2 German Norm S 10-4-3-10.

venting grain coarsening, which result cannot be achieved by the use ofmolybdenum alone. Specifically, in the processing of the steel it isaustenitized at a high tem perature on the order of 2200 F. and thenhardened during cooling. The austenitizing step involves heating todissolve the carbide-forming elements. After quenching fromaustenitizing temperature, the material is subjected to reheating at alower temperature at which the carbideforming elements are precipitatedin the form of carbides. This, of course, produces the desired secondaryhardening. During austenitizing, the carbon is dissolved in theaustenite, which upon cooling transforms to a required hardcarbon-containing martensite. The carbideforming elements remain insolution in the martensite. Subsequently, however, the carbide-formingelements during tempering combine with the carbon in the steel and formcarbides. This carbide precipitation results in the desired secondaryhardening. The cobalt present in the alloy contributes to the retentionof hardness at high temperatures. As above described, the presence ofnitrogen in an amount of at least .03%, and preferably within the rangeof .03 to 08%, is necessary to achieve a fine carbide distribution. Thisresult of nitrogen has been found not to increase significantly atnitrogen levels substantially above .08%. It should be noted that themaximum amount of nitrogen present in the alloy is limited by thesolubility of nitrogen in the melt, unless the nitrogen is The Rex 71P/M materials were made from particles of the alloy of a mesh size of-+325 US. Standard. A charge of these particles was placed into a mildsteel cylinder about 4 in. long and having a 3% in. diameter. Thiscontainer. which was gas tight, was heated to a temperature of 2100 F.for about 4 hours at which time the container interior was connected toa pump which was used to remove the gaseous reaction products from thecontainer. The container, at a temperature of about 2000 F., was placedin a die and a ram of a ZOO-ton press was used to compact the containerand charge to a density greater than 95%. After compacting, the materialwas forged into 4 in. square bars, during which operation a density ofessentially 100% was achieved. The other steels, as reported in Table I,were conventionally cast and wrought from 50-pound. air-induction heats.Specifically, they were cast into 4 x 4 x 10 in. ingots and forged toin. bars as were the above samples produced by the described powdermetallurgy technique. All of the steels reported in Table I wereaustenitized at a temperature of about 2200 F. for 4 mins. and oilquenched. The steels of Table I were tested for machinability by theconventional Drill Machinability Test. In this test A: in. drills wereused to drill holes 0.250 in. deep while operating at 460 r.p.m. using aconstant thrust at the quill of 150 pounds.

As may be seen from the results presented in Table II, the Rex 71 P/Msample while having a hardness of TAB LE IL-MACHINABILITY Average time(sec required to drill four Machina- I-Iardness -1IL holes bility cindex,

(annealed Drill Drill Drill Drill Drill Drill M.I.

Steel stock) No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 percent RQX 49 21 29. 626. 8 25. 4 25. 4 27. 0 23. 3 100 Rex M42... 21.5 26.6 23.6 21.7 114 Rex71 P/M 26 23.0 23. 7 20. 9 20.3 20. 8 18. 7 124 1 M I average time todrill standard (Rex 49) average time to drill test material added bygaseous diifussion as above described. The principal role of chromium inthe alloy is to delay the precipitation of carbides upon tempering tocontribute to the high-temperature hardness.

It may be seen, therefore. that the combination of nitrogen and tungstenis critical for the purpose of preventing carbide growth andagglomeration and hence grain coarsening; whereas, vanadium provides thewear-resistance carbides necessary for good tool life.

To demonstrate the present invention samples of the steels with thecomposition listed in Table I were pro- Continuous-Cut Lathe TurningTest results reported in Table 111.

TABLE 11I.CONTINUOUS-CUI LATHE TURNING 1 Average tool life 2 in minutesat indicated cutting speed Feed 0.010 in./rev.; depth-ofcut 0.062-in.;cutting oil: none; tool ge. ometry: 3, 6, 10, 10, 10, 10; 0,030-in. noseradius.

Complete tool nose failure.

3 140,000 p.s.i. tensile strength.

It may be seen from the test results of Table III that the average toollife of the Rex 71 P/M lathe cutting tools is four times that of Rex 49(M41) during use in the test to continuously turn the reporteddifiicult-tomachine alloys at identical speed, feed, and depth-ofcut.Specifically, as shown in Table III, the Rex 71 P/ M cutting toolsaveraged 16 minutes before failure in cutting at 35 s.f.p.1n. aworkpiece of AISI H13 die steel having a hardness of 53 R whereas, thebest performance of tools made from conventional high-perf0rrnancehigh-speed steels was an average of 2.8 mins. for M4] and 6.2 mins. forM42 cutting tools used to cut the same workpiece. A cutting tool of thesteel of the invention also showed superior performance when comparedwith cutting tools of conventional tool steels in cutting a workpiece ofC125 AVT titanium. In this application, as shown in Table III, the Rex71 P/ M cutting tool of the invention averaged 86 mins. before failure;whereas, the cutting tools made from M42 and M41 averaged 53.8 mins. and22.7 mins., respectively, before failure.

from the atmosphere. The high-nitrogen Steels E, D and F were meltedwith ferrochromium-containing nitrogen. Before heat-treating, all thesteels of Table IV were spheroidize annealed at 1600 F. for two hours,cooled to 1400 F., held for 4 hours, and then air-cooled to roomtemperature, Laboratory size specimens cut from these bar samples wereaustenitized at IO-degree intervals between 2200 F. and 2270 F. andthereafter oil quenched. The grain-coarsening characteristics of theasquenched microstructures were determined.

A metallographic examination of the samples, which were austenitized attemperatures between 2200 F. and 2270 F showed that the high nitrogenSteels B, D, and F retained a fine grain structure in the presence ofhigher temperatures than did the nitrogen-free Steels A, C, E, and Gwith an equivalent interstitial alloy content. This comparison betweenthe high-nitrogen steels and the nitrogen-free steels is shown in FIGS.2A and 2B. In both of these figures a three-dimensional plot of grainsize vs. austenitizing temperature and total interstitial content ispresented. In FIG. 2A the total interstitial content consists of carbon;whereas, with FIG. 2B the interstitial content consists of carbon plusnitrogen. The range of total interstitial content is from .85 to 1.10%.It may be seen from the results presented in this figure that althoughgrain size increases both with and without nitrogen in the presence ofincreased austenitizing temperatures, a nitrogen addition, within thescope of the present invention, drastically depresses thisgrain-coarsening effect. For example, with the total interstitialcontent being equal in the absence of nitrogen an austenitizingtemperature of 2240 F. results in a grain size of 9 Snyder- Gratf;whereas, in the presence of nitrogen an austenitizing temperature of2240" F. results in a grain size of 13 Snyder-Grail.

I claim:

1. A tool steel characterized by a combination of good cuttingperformance and machinability consisting essentially of, in weightpercent, carbon 1 to 1.4, chromium TABLE IV.CHEMICAL COMPOSITION OFEXPERIMENTAL STEELS Molybdenum high-speed steels, composition, weightpercent C N M11 S P Si V W 0. 86 0. 01 0. 37 0. 0l9 0. 010 0. 35 3. 87l. 75 l. 75 0. 85 0. 06 0. 3O 0. 018 0. 015 0. 29 3. 74 2. l1 1. 80 1.00 0. 01 0. 31 0. l3 0. 014 0. 4. 00 2. 13 l. 66 0. 91 0. 08 0. 0. 13 0.016 0. 28 3. 95 2. 29 1.66 1. O9 0. 01 0. 25 0. 020 0. 020 0. 27 3. 752. 05 1. 75 U. 98 0. 08 0. 24 0. 020 0. 020 0. 35 3. 75 2. 05 1. 75 0.94 0. 01 0. 54 0. 020 0. 020 0. 25 3. 76 2. 05 1. 75

To demonstrate the criticality of nitrogen within the ranges of thepresent invention in controlling carbide form, size, and distribution,steels of the compositions reported in Table IV were produced. In thesesteels the tungsten content, in particular, was maintained at a lowlevel so that its effect with regard to grain refinement could besubstantially discounted.

All the steels reported in Table IV were melted as -pound inductionheats, teemed into 4-inch square ingot molds and hot forged to %-inchsquare bars. The melting charges of Steels A, C, E and G containedhigh-purity electrolytic chromium to limit the nitrogen content to 0.01%or less. Melting and teeming were carried out under a protected argonblanket to prevent nitrogen absorption 4 to 6, vanadium 1 to 1.5,tungsten 7.5 to 13, molybdenum 3.5 to 7, cobalt 9 to 15, nitrogen about.03 to .08

and balance iron.

References Cited UNITED STATES PATENTS 2,983,601 5/1961 Fletcher 75-126H 3,012,879 12/1961 Schemp 75-126 H 3,113,862 12/1963 Harvey 7--l26 CHYLAND BIZOT, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT QFFICE Q IIICAIE ECTION Patent No. 3 75 Dated D c14, 1971 Inventor(s) G ry Steven It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 15, change "again" to grain;

Column Table II, change "l/2-=-in,, holes" to --1/ L-1n. holes--;

Column 5, Table IV, under column headed "W" first line change "1.75" to--l,85-- (first occurrence under "w") Signed and sealed this 27th day ofJune 1972..

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents USCOMM-DC 5Q376-P69 U.S. GOVERNMENT PRINTING OFFICE: I5690-365-334 ORM PO-1050 (10-69) UNITED STATES PATENT OFFICE QETMQATE s s3,627,5l L Dated Dec a 1 1, 1971 Patent No.

Inventor(s) G ry Steven It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 15, change "again" to --grain--;

Column l, Table II; change "l/E-in, holes" to --1/ -in. holes-==-=;

Column 5, Table IV, under column headed "w" first line change "1.75 to-l.85-- (first occurrence under "W") Signed and sealed this 27th day ofJune 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHAIK Attesting Officer Commissionerof Patents RM PO'TOSO (10459) USCOMM-DC 60376-1 69 U.S. GOVERNMENTPRINTING OFFICE: I969 0366334

